DH1 has a simple amp in it, I use it as-is with my 555's. However, you can't crank them past my personal pain threshold (which is good, I imagine), but maybe some people would want a secondary amp.

Even at below your pain threshold, you can still lose your hearing quite easily with extended usage.

If you're really going to fork over for an amp though, don't put it in a chain with something like the DH1. An amp is about clean power and fidelity, and is only as good as its source. For the price of the DH1, I'd imagine the quality of the output is going to work against whatever niceties that a quality amp will provide. Generally: the fewer breaks in the signal chain, the better.